The optical transmission medium having the distribution of refractive index varying continuously in a fixed direction, are widely used as bar lenses having the function of convex lens, bar lenses having the function of concave lenses and broad band optical transmission fibers. Among them, the optical transmission media made of transparent synthetic resins are more widely used in recent years because they have several advantages in the lightness in weight, economy, easy handling, high impact resistance and flexibility, as compared with those made of quartz.
The optical transmission media are roughly classified into a single mode type and a multimode type in view of the modes of transmission of light rays. The radius of core of multimode type medium is sufficiently large as compared with the wave length of light ray to be transmitted. Accordingly, it can transmit light rays of a plurality of modes. The multimode type media are further divided, according to the mode of gradient of refractive index, into a step index type in which the refractive index is varied step by step between the core and clad and a graded index type in which the refractive index is gradually and continuously varied.
Concerning the method of producing plastics-made optical transmission media of multimode graded index type by polymerization in specific reaction vessels, there are proposed the following methods in the conventional art.
In Japanese Patent Publication No. 52-5857 (U.S. Pat. No. 3,955,015), a monomer to form a polymer of a different refractive index is subjected to diffusional transfer into a specific transparent solid substance in a polymerization process having a previously formed three-dimensional network structure. After that, the whole polymerization reaction is stopped to obtain an optical transmission medium of refractive index distribution type.
In this method, however, it is necessary that the transparent solid substance is previously made into three-dimensional network structure using a multifunctional radical polymerizable monomer, in order to maintain the configuration of the transparent solid substance. For this reason, it must previously be made separately, which costs much labor. In addition, the obtained polymer of three-dimensional network structure is not good in thermo-plasticity and it is not suitable for drawing and other post-forming processes. In other words, practical plastics-made optical transmission fibers should have stiffness and tensile strength as fiber materials in the drawing step of manufacturing process. However, the optical transmission medium prepared through the above process has inherently three-dimensional network structure, so that it is not suitable for drawing.
In Japanese Patent Publication No. 54-30301 and Japanese Laid-Open Patent Publication No. 61-130904 are proposed methods for producing optical transmission media having refractive index gradients paying attention to the difference between the monomer reactivity ratios: r.sub.1 and r.sub.2 of two kinds of monomers.
In the above methods utilizing the difference between the monomer reactivity ratios of monomers, it is desirable that the difference between the radical copolymerization reactivity ratios of r.sub.1 and r.sub.2 is large, as a result, the formation of homopolymer occurs at first and macromolecules of homopolymer are formed with causing phase separation, which sometimes makes the obtained optical transmission medium clouded to reduce the optical transmission efficiency.
When too large monomer reactivity ratios are selected, one of monomers must be the one having a low rate of polymerization such as vinyl benzoate and vinyl o-chlorobenzoate used in the example of Japanese. Patent Publication No. 54-30301, and vinyl phenylacetate used in the example of Japanese Laid-Open Patent Publication No. 61-130904. The use of monomers whose monomer reactivity ratios are largely different means that monomers of considerably low reactivity are used in copolymerization. As a result, the monomer having high reactivity is firstly polymerized and the monomer having low reactivity remains in a high concentration in the final stage of polymerization. Thus, it takes much time to complete the polymerization and, in an extreme case, the removal of remaining monomer is required.
In addition, the existence of residual monomer causes several undesirable influences on mechanical characteristics of transmission media such as tensile strength, elongation and stiffness, and the long term stability of transmission media owing to the postpolymerization or decomposition of residual monomer.
The present inventors have carried out extensive investigations concerning the process to form copolymer resins by radical reaction. When the viscosity of monomer liquid rises and the liquid turns into gel with the progress of the polymerization of monomers, the growing polymer radical is hardly diffused in the gel because the molecular weight of the polymer radical is large. In this case, the termination reaction in the radical reaction between two molecules of growing polymer radicals hardly proceeds, .as a result, the rate of polymerization increases. In this state, in order to propagate the growing polymer radical further by polymerization, it is necessary that the starting monomer is diffused in the gel and the growing polymer radical is continuously supplied with the starting monomer.
The above phenomenon is accepted as the so-called gel effect in the radical polymerization. When radical polymerization is carried out so that the gel effect is produced from any terminal point of a reaction liquid in the reaction vessel, the polymerization proceeds in a certain direction from an optional point step by step and finally the progress of the polymerization reaches the other end portion.
The present inventors have made further investigations concerning the process of polymerizing a monomer .in a vessel made of a polymer which can be dissolved in the monomer to be polymerized. More particularly, the polymer in the wall of the vessel is dissolved into the monomer and the dissolved polymer is selectively discharged from the gel. During the polymerization, the dissolving of the vessel wall is continued and it is discharged out of the gel. The concentration of the discharged polymer is gradually lowered as the distance from the wall increases. As the result, the distribution of composition of an obtained polymer is continuously varied in the direction of the progress of polymerization relative to the fed monomer.
In view of the above-described facts, the inventors of the present application have accomplished a method of manufacturing an optical transmission medium made of a synthetic resin with a novel method of copolymerization.
The present invention can eliminate several disadvantages in the methods of manufacturing optical transmission media of refractive index distribution type by the conventional polymerization process, and with utilizing the novel findings on the states of polymerization, the present invention attains the object to provide a method of manufacturing a multimode optical transmission medium having excellent continuous distribution of refractive index with high productivity.